Non-impact printers used in information processing equipment are well known at present. These machines include a recording carrier, most often comprising a rotary drum or an endless belt, on the surface of which sensitized zones, also known as latent images, can be formed electrostatically or magnetically, corresponding to the characters or images to be printed. These latent images are then developed, or in other words made visible, with the aid of a powdered developer, which when deposited on the recording carrier is attracted only by the sensitized zones on it, thus forming an image in powder on the surface of the carrier. After that, the recording carrier is put in contact with a sheet of paper to enable the developer particles comprising the powdered image to be transferred to the sheets and definitively fixed there.
The application of developer particles to the recording carrier in printing machines of this type is accomplished by applicator devices of a known type, such as that described in U.S. Pat. No. 4,246,588 (corresponding to French Pat. No. 2.408.462). With these applicator devices, however, despite all the care taken in their construction, it is difficult to prevent the developer particles from being deposited not only to excess over the sensitized zones of the recording carrier but also, although in very small amounts, outside these zones. For this reason the printers are also provided with a retouching device, which is disposed between the particle applicator device and the station where the particles are transferred to a sheet of paper and makes it possible to remove the excess developer particles located on the surface of the recording carrier. Although retouching devices capable of performing the retouching by magnetic attraction or by blowing air have been made, the preference at present is for retouching devices that function by air suction and have the advantage of being non-polluting and of enabling removal of the excess particles on the recording carrier surface, without requiring that the particles have magnetic properties in order to accomplish this. Hence a retouching device has been embodied as described in U.S. Pat. 3,680,528, which includes both a suction conduit provided with a slit or nozzle, extending in proximity with the surface of the recording carrier, and an opening connected via a duct to a suction turbine. In this device, the air aspirated via the slit in the suction conduit entrains the excess particles located on the portions of the recording carrier located vertically of that slit. The air thus laden with particles circulates in the duct, and then having passed through the turbine is turned back toward an outlet conduit to the end of which a recovery bag, made of some material permeable to air, has been affixed. The air that is returned via the turbine can thus pass through the bag and be returned to the atmosphere, while the developer particles that were transported by this flow of air are stopped by the mesh of the bag and can then be recovered and later replaced in the applicator device. In this retouching device, however, because the air that passes through the suction turbine is necessarily laden with developer particles, over the long term the turbine is fouled, which makes this retouching device particularly tedious to maintain.
To overcome this disadvantage, a retouching device as described in U.S. Pat. No. 4,046,682 has been proposed, in which an endless belt of a material permeable to air and thus acting as a filter, passes in the course of its path through the duct connecting the suction conduit to the suction turbine. Under these conditions, the developer particles that are transported by the air aspirated by the turbine are stopped in their passage by the endless belt and hence cannot pass through the turbine. Nevertheless, this is not entirely satisfactory, because not only is the recovery of the particles captured by the belt relatively impractical and never complete, but also the portion of the duct that is located between the belt and the suction nozzle finally becomes more or less obstructed, markedly diminishing the output of the suction turbine.